Tight junctions, or zonula occludens, are structures which allow for strict regulation of passage of material between cells. Dysregulation at tight junctions has severe consequences for the cell and tissue environment and is present in a number of diseases and disorders such as diabetic macular edema.
A number of proteins have been characterized as present in tight junctions, including for instance occludin, claudin, JAM, ZO-1, 2 and 3, MAGI-1, 2, and 3, PAR3/6 and MUPP1. Regulation of these proteins, their levels, localization, and structural characteristics, may play a role in normal functioning as well as in disease states of an organism. Post-translation modification, such as phosphorylation, is a mechanism of protein regulation.
Vascular Endothelial Growth Factor (VEGF) has been shown to stimulate phosphorylation of at least one tight junction protein, occludin, with consequent induction of vascular permeability. The action of VEGF is mediated, in part, by protein kinase C (PKC) beta.
PKC designates a class of kinases which play central roles in key cell signaling processes such as gene expression and regulation of cell growth. There are numerous isoforms of PKC and these are typically classified as: 1) “calcium-dependent” conventional isoforms which are regulated by both calcium and diacylglycerol, such as PKC-beta; 2) “calcium-independent” novel isoforms which are regulated by diacylglycerol but do not require calcium, such as PKC-delta; and 3) “atypical” isoforms which do not require calcium for activation and which are not regulated by diacylglycerol. PKC zeta is an “atypical” PKC isoform.
In this context, it is of interest that protein kinase C beta inhibitors are currently undergoing phase 3 clinical trials for the treatment of macular edema in diabetic retinopathy. However, it is found that inhibition of PKC beta prevents only about 50% of VEGF induced endothelial permeability in primary retinal endothelial cell culture.
Thus, there is a continuing need for compositions including a PKC inhibitor which regulates tight junction permeability and methods for regulating tight junction permeability in healthy and diseased cells and tissues.
Furthermore, protein kinase C isoforms are present in a variety of cell types and have been associated with a number of pathological diseases and disorders. There is a continuing need for compositions and methods of regulating PKC activity in vitro and in vivo.